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<div class="titlepage"><div><div><h5 class="title">
<a name="spirit.qi.reference.auxiliary.eps"></a><a class="link" href="eps.html" title="Epsilon Parser (eps)">Epsilon Parser
          (<code class="computeroutput"><span class="identifier">eps</span></code>)</a>
</h5></div></div></div>
<h6>
<a name="spirit.qi.reference.auxiliary.eps.h0"></a>
            <span class="phrase"><a name="spirit.qi.reference.auxiliary.eps.description"></a></span><a class="link" href="eps.html#spirit.qi.reference.auxiliary.eps.description">Description</a>
          </h6>
<p>
            The Epsilon (<code class="computeroutput"><span class="identifier">eps</span></code>) is
            a multi-purpose parser that returns a zero length match.
          </p>
<h6>
<a name="spirit.qi.reference.auxiliary.eps.h1"></a>
            <span class="phrase"><a name="spirit.qi.reference.auxiliary.eps.simple_form"></a></span><a class="link" href="eps.html#spirit.qi.reference.auxiliary.eps.simple_form">Simple
            Form</a>
          </h6>
<p>
            In its simplest form, <code class="computeroutput"><span class="identifier">eps</span></code>
            matches the null string and always returns a match of zero length:
          </p>
<pre class="programlisting"><span class="identifier">eps</span> <span class="comment">// always returns a zero-length match</span>
</pre>
<p>
            This form is usually used to trigger a semantic action unconditionally.
            For example, it is useful in triggering error messages when a set of
            alternatives fail:
          </p>
<pre class="programlisting"><span class="identifier">r</span> <span class="special">=</span> <span class="identifier">a</span> <span class="special">|</span> <span class="identifier">b</span> <span class="special">|</span> <span class="identifier">c</span> <span class="special">|</span> <span class="identifier">eps</span><span class="special">[</span><span class="identifier">error</span><span class="special">()];</span> <span class="comment">// Call error if a, b, and c fail to match</span>
</pre>
<h6>
<a name="spirit.qi.reference.auxiliary.eps.h2"></a>
            <span class="phrase"><a name="spirit.qi.reference.auxiliary.eps.semantic_predicate"></a></span><a class="link" href="eps.html#spirit.qi.reference.auxiliary.eps.semantic_predicate">Semantic
            Predicate</a>
          </h6>
<p>
            Semantic predicates allow you to attach a conditional function anywhere
            in the grammar. In this role, the epsilon takes a <a class="link" href="../basics.html#spirit.qi.reference.basics.lazy_argument">Lazy
            Argument</a> that returns <code class="computeroutput"><span class="keyword">true</span></code>
            or <code class="computeroutput"><span class="keyword">false</span></code>. The <a class="link" href="../basics.html#spirit.qi.reference.basics.lazy_argument">Lazy
            Argument</a> is typically a test that is called to resolve ambiguity
            in the grammar. A parse failure will be reported when the <a class="link" href="../basics.html#spirit.qi.reference.basics.lazy_argument">Lazy
            Argument</a> result evaluates to <code class="computeroutput"><span class="keyword">false</span></code>.
            Otherwise an empty match will be reported. The general form is:
          </p>
<pre class="programlisting"><span class="identifier">eps</span><span class="special">(</span><span class="identifier">f</span><span class="special">)</span> <span class="special">&gt;&gt;</span> <span class="identifier">rest</span><span class="special">;</span>
</pre>
<p>
            The <a class="link" href="../basics.html#spirit.qi.reference.basics.lazy_argument">Lazy Argument</a>
            <code class="computeroutput"><span class="identifier">f</span></code> is called to do a semantic
            test (say, checking if a symbol is in the symbol table). If test returns
            true, <code class="computeroutput"><span class="identifier">rest</span></code> will be evaluated.
            Otherwise, the production will return early with a no-match without ever
            touching rest.
          </p>
<h6>
<a name="spirit.qi.reference.auxiliary.eps.h3"></a>
            <span class="phrase"><a name="spirit.qi.reference.auxiliary.eps.header"></a></span><a class="link" href="eps.html#spirit.qi.reference.auxiliary.eps.header">Header</a>
          </h6>
<pre class="programlisting"><span class="comment">// forwards to &lt;boost/spirit/home/qi/auxiliary/eps.hpp&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">spirit</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">qi_eps</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
</pre>
<p>
            Also, see <a class="link" href="../../../structure/include.html" title="Include">Include Structure</a>.
          </p>
<h6>
<a name="spirit.qi.reference.auxiliary.eps.h4"></a>
            <span class="phrase"><a name="spirit.qi.reference.auxiliary.eps.namespace"></a></span><a class="link" href="eps.html#spirit.qi.reference.auxiliary.eps.namespace">Namespace</a>
          </h6>
<div class="informaltable"><table class="table">
<colgroup><col></colgroup>
<thead><tr><th>
                    <p>
                      Name
                    </p>
                  </th></tr></thead>
<tbody><tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">eps</span> <span class="comment">// alias:
                      boost::spirit::qi::eps</span></code>
                    </p>
                  </td></tr></tbody>
</table></div>
<h6>
<a name="spirit.qi.reference.auxiliary.eps.h5"></a>
            <span class="phrase"><a name="spirit.qi.reference.auxiliary.eps.model_of"></a></span><a class="link" href="eps.html#spirit.qi.reference.auxiliary.eps.model_of">Model
            of</a>
          </h6>
<div class="blockquote"><blockquote class="blockquote"><p>
              <a class="link" href="../parser_concepts/primitiveparser.html" title="PrimitiveParser"><code class="computeroutput"><span class="identifier">PrimitiveParser</span></code></a>
            </p></blockquote></div>
<div class="variablelist">
<p class="title"><b>Notation</b></p>
<dl class="variablelist">
<dt><span class="term"><code class="computeroutput"><span class="identifier">f</span></code></span></dt>
<dd><p>
                  A <a class="link" href="../basics.html#spirit.qi.reference.basics.lazy_argument">Lazy
                  Argument</a> that evaluates <code class="computeroutput"><span class="keyword">bool</span></code>.
                </p></dd>
</dl>
</div>
<h6>
<a name="spirit.qi.reference.auxiliary.eps.h6"></a>
            <span class="phrase"><a name="spirit.qi.reference.auxiliary.eps.expression_semantics"></a></span><a class="link" href="eps.html#spirit.qi.reference.auxiliary.eps.expression_semantics">Expression
            Semantics</a>
          </h6>
<p>
            Semantics of an expression is defined only where it differs from, or
            is not defined in <a class="link" href="../parser_concepts/primitiveparser.html" title="PrimitiveParser"><code class="computeroutput"><span class="identifier">PrimitiveParser</span></code></a>.
          </p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                    <p>
                      Expression
                    </p>
                  </th>
<th>
                    <p>
                      Semantics
                    </p>
                  </th>
</tr></thead>
<tbody>
<tr>
<td>
                    <p>
                      <code class="computeroutput"><span class="identifier">eps</span></code>
                    </p>
                  </td>
<td>
                    <p>
                      Match an empty string (always matches).
                    </p>
                  </td>
</tr>
<tr>
<td>
                    <p>
                      <code class="computeroutput"><span class="identifier">eps</span><span class="special">(</span><span class="identifier">f</span><span class="special">)</span></code>
                    </p>
                  </td>
<td>
                    <p>
                      If <code class="computeroutput"><span class="identifier">f</span></code> evaluates
                      to <code class="computeroutput"><span class="keyword">true</span></code>, return
                      a zero length match.
                    </p>
                  </td>
</tr>
</tbody>
</table></div>
<h6>
<a name="spirit.qi.reference.auxiliary.eps.h7"></a>
            <span class="phrase"><a name="spirit.qi.reference.auxiliary.eps.attributes"></a></span><a class="link" href="eps.html#spirit.qi.reference.auxiliary.eps.attributes">Attributes</a>
          </h6>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                    <p>
                      Expression
                    </p>
                  </th>
<th>
                    <p>
                      Attribute
                    </p>
                  </th>
</tr></thead>
<tbody><tr>
<td>
                    <p>
                      <code class="computeroutput"><span class="identifier">eps</span></code>
                    </p>
                  </td>
<td>
                    <p>
                      <code class="computeroutput"><span class="identifier">unused</span></code>
                    </p>
                  </td>
</tr></tbody>
</table></div>
<h6>
<a name="spirit.qi.reference.auxiliary.eps.h8"></a>
            <span class="phrase"><a name="spirit.qi.reference.auxiliary.eps.complexity"></a></span><a class="link" href="eps.html#spirit.qi.reference.auxiliary.eps.complexity">Complexity</a>
          </h6>
<div class="blockquote"><blockquote class="blockquote"><p>
              For plain (<code class="computeroutput"><span class="identifier">eps</span></code>) the
              complexity is O(1). For Semantic predicates (<code class="computeroutput"><span class="identifier">eps</span><span class="special">(</span><span class="identifier">f</span><span class="special">)</span></code>) the complexity is defined by the
              function <code class="computeroutput"><span class="identifier">f</span></code>.
            </p></blockquote></div>
<h6>
<a name="spirit.qi.reference.auxiliary.eps.h9"></a>
            <span class="phrase"><a name="spirit.qi.reference.auxiliary.eps.example"></a></span><a class="link" href="eps.html#spirit.qi.reference.auxiliary.eps.example">Example</a>
          </h6>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
              The test harness for the example(s) below is presented in the <a class="link" href="../basics.html#spirit.qi.reference.basics.examples">Basics Examples</a>
              section.
            </p></td></tr>
</table></div>
<p>
            Some using declarations:
          </p>
<p>
</p>
<pre class="programlisting"><span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">qi</span><span class="special">::</span><span class="identifier">eps</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">qi</span><span class="special">::</span><span class="identifier">int_</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">qi</span><span class="special">::</span><span class="identifier">_1</span><span class="special">;</span>
<span class="keyword">namespace</span> <span class="identifier">phx</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">phoenix</span><span class="special">;</span>
</pre>
<p>
          </p>
<p>
            Basic <code class="computeroutput"><span class="identifier">eps</span></code>:
          </p>
<p>
</p>
<pre class="programlisting"><span class="identifier">test_parser</span><span class="special">(</span><span class="string">""</span><span class="special">,</span> <span class="identifier">eps</span><span class="special">);</span> <span class="comment">// always matches</span>
</pre>
<p>
          </p>
<p>
            This example simulates the "classic" <code class="computeroutput"><span class="identifier">if_p</span></code>
            parser. Here, <code class="computeroutput"><span class="identifier">int_</span></code> will
            be tried only if the condition, <code class="computeroutput"><span class="identifier">c</span></code>,
            is true.
          </p>
<p>
</p>
<pre class="programlisting"><span class="keyword">bool</span> <span class="identifier">c</span> <span class="special">=</span> <span class="keyword">true</span><span class="special">;</span> <span class="comment">// a flag</span>
<span class="identifier">test_parser</span><span class="special">(</span><span class="string">"1234"</span><span class="special">,</span> <span class="identifier">eps</span><span class="special">(</span><span class="identifier">phx</span><span class="special">::</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">c</span><span class="special">)</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">)</span> <span class="special">&gt;&gt;</span> <span class="identifier">int_</span><span class="special">);</span>
</pre>
<p>
          </p>
<p>
            This example simulates the "classic" <code class="computeroutput"><span class="identifier">while_p</span></code>
            parser. Here, the kleene loop will exit once the condition, <code class="computeroutput"><span class="identifier">c</span></code>, becomes true. Notice that the condition,
            <code class="computeroutput"><span class="identifier">c</span></code>, is turned to <code class="computeroutput"><span class="keyword">false</span></code> when we get to parse <code class="computeroutput"><span class="number">4</span></code>.
          </p>
<p>
</p>
<pre class="programlisting"><span class="identifier">test_phrase_parser</span><span class="special">(</span><span class="string">"1 2 3 4"</span><span class="special">,</span>
    <span class="special">*(</span><span class="identifier">eps</span><span class="special">(</span><span class="identifier">phx</span><span class="special">::</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">c</span><span class="special">)</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">)</span> <span class="special">&gt;&gt;</span> <span class="identifier">int_</span><span class="special">[</span><span class="identifier">phx</span><span class="special">::</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">c</span><span class="special">)</span> <span class="special">=</span> <span class="special">(</span><span class="identifier">_1</span> <span class="special">==</span> <span class="number">4</span><span class="special">)]));</span>
</pre>
<p>
          </p>
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<td align="right"><div class="copyright-footer">Copyright © 2001-2011 Joel de Guzman, Hartmut Kaiser<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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